Diagnostic work would need to be done to confirm, but I'd suspect a worn valve body bore that is causing your torque converter to slip. This results in excessive fluid heat which the compter sees and responds to by putting the transmission into "failsafe" mode. Failsafe mode maxes internal line pressures so shifting becomes VERY harsh. It also reduces cooler flow so the fluid gets even hotter...
If you keep driving it, you risk frying the torque converter clutch and cracking the forward drum.
Repair depends on where you are in the failure cycle. Flushing the fluid won't fix this problem.
Look for posts by mercman from about three years ago for more in-depth description.
If I ever get my VB and solenoid pack in that I bought off ebay, I'll let ya know if my tranny is fixed....
